Outboard end units have been known which are provided with a supercharger for boosting engine power output, and one example of such outboard end units is disclosed in Japanese Patent Application Laid-Open Publication No. HEI-05-141260 (hereinafter referred to as “the relevant patent literature”).
As shown in FIG. 6 hereof, an engine of the outboard end unit disclosed in the relevant patent literature includes a crankcase 101, a cylinder body 102, a cylinder head 103, a crankshaft 104, a connecting rod (con rod) 105 and a piston 106. Fuel is combusted in a combustion chamber 107, and resultant exhaust of high temperature is discharged through an exhaust port 108. The engine further includes a supercharger 109 that force-feeds air into the combustion chamber 107 via a connecting pipe 110. Because the supercharger 109 gets hot during the force-feeding of air, it has to be cooled.
Cooling water is introduced via a water jacket 111, delivered through another water jacket 112 to still another water jacket 113, provided in the supercharger 109, to cool the supercharger 109, and then delivered through still other water jackets 114 and 115 to a further water jacket 116 to cool the cylinder body 102 and cylinder head 103, after which the cooling water is discharged.
Note that the engine cylinder block 102 and cylinder head 103 (i.e., engine-side components) and the supercharger 109 differ from each other in required cooling temperature range. However, in the outboard engine unit disclosed in the relevant patent literature, the supercharger 109 and the engine-side components are cooled by one and the same cooling system, and thus, it is difficult to appropriately adjust the temperatures of the supercharger 109 and the engine-side components independently of each other. Further, because the supercharger 109 that gets hot is provided within an engine cover, separate cooling and heat shielding structures are required for the supercharger 109, which would unavoidably lead to an increased size of the outboard engine unit.